Frequency discriminator network



Feb. 8, 1944. J. D. REID 2,341,240

FREQUENCY DI S CRIMINATOR NETWORK Filed Aug. 17, 1940 2 Sheets-Sheet l ZSnventor 2 ttomeg Feb. 8, 1944. J. D. REID FREQUENCY DISCRIMINATOR NETWORK Filed Aug. 17, 1940 2 Sheets-Sheet 2 M Q 6 F w Q I il.ii u u M m 0 0 7 z w w 70 v u m (H iwlfw bl ||||||i m FIPEQ.

Ennentor Patented Feb. 8, 1944 FREQUENCY DISCRIMINA'I'OR NETWORK John 1). Reid, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application August 17, 1940, Serial No. 353,028

13 Claims. (01. 250-27) This invention relates to the detection of Hequency-modulated impulses, and has for its principal object the provision of an improved apparatus and method of operation for providing frequency-modulation detection and frequencyresponsive controlling potentials.

Frequency variation detector systems or frequency discriminator networks are disclosed, for example, in the United States patent to Conrad 2,057,640 or Seeley 2,121,103.

In the former, the frequency discriminator network comprises two tuned signal circuits, one tuned above and the other tuned below a carrier or center frequency, and connected with suitable rectifiers back-to-back to provide cumulative response to the frequency variation of an incoming wave, and difierential response to the amplitude variations on said wave. :Another arrangement is provided in Seeley, involving two tuned circuits connected in phase opposition through suitable rectifiers for the same purpose.

Frequency discriminator networks of this type are used in connection with automatic frequency control (AFC) means for superheterodyne receiver oscillators and in detectors for frequency modulation receiving systems.

It is a primary object of the invention to provide an improved means and method for obtaining cumulative response to frequency variation of a carrier wave and difierential response to amplitudevariation on said wave.

It is an object of the present invention to provide an improved frequency discriminator network, involving one tuned circuit and one tuning element therein, which may operate to shift the discriminator frequency over a relatively wide frequency range, thereby permitting rapid and accurate adjustments in manufacture of apparatus incorporating the network, and providing means for continuously variable tuning in receiving sys tems covering relatively wide frequency ranges for the reception of frequency-modulated signals I and the like, and, at the same time, utilizing a minimum of circuits and variable elements therein.

It is also an object of this invention to provide an improved frequency discriminator network for demodulating frequency-modulated carrier waves, and to detect or measure frequency variations of electrical oscillations or of a carrier wave, the frequency of which is varied or modulated in accordance with a signal, and to develop control and signal potentials therefrom.

It is a further object of the present invention to provide an improved frequency discriminator network of the two-terminal type providing one tuned shunt path acrossva signal-conveying channel, the said path being in effect parallel-resonant at one frequency and in its entirety being series-resonant at another frequency, on opposite sides of amean carrier or center frequency, together with suitable rectifier means for deriving therefrom output voltages representing demodulated frequency-modulated signals, and/or control potentials responsive to changes in frequency of received signals. In a preferred arrangement, the said output voltages are made equal and opposite at the mean or center frequency, thereby providing substantially zero D. C. (direct current) or output voltage at said frequency.

In accordance with the invention, a high or intermediate frequency signal-conveying channel for a signal-receiving system, such as the intermediate frequency amplifier of a superheterodyne receiver, is provided with an output circuit including a two-terminal frequency discriminator network providing a single shunt path across the channel and including a parallelresonant circuit and means providing a capacity in series therewith effective to render the shunt path series-resonant, together with rectifier means and output circuits therefor connected with said shunt path at suitable points for deriving controlling and output potentials therefrom resulting from the frequency variation of an applied signal.

HeretOfore it has been considered desirable or necessary to provide two signal paths or two tuned circuits in a frequency discriminator network. In accordance with the present invention, however, the necessity for two tuned circuits or two signal paths has been avoided, and the tuning and operation of the system is greatly simplified in that but one tuned circuit is provided, involving both parallel and series-resonance of the elements therein to effect a response to frequency variation from a mean, center or midband .frequency. At the same time, an improvement in sensitivity, selectivity and stability over that of known circuits is obtained, and the improved sensitivity, selectivity and stability may be obtained at high and ultra high signal frequencies. Heretofore, known discriminator circuits have been lacking in one or more of these qualities.

In a frequency-modulation detector, it is desirable, as is well-known, to cause th peaks of the frequency-response characteristic thereof to be maintained relatively close together for maximum sensitivity and selectivity and, accordingly, it is a still further object of this invention to provide a frequency discriminator network wherein a single tuned signal path, seriesand parallelresonant to differing frequencies, may include. as the series capacity element thereof for series resonance, the inherentinterelectrode capacity of a tube or rectifier associated with the circuit for deriving control potentials therefrom. In other words, a discriminator network is herein provided in which the peaks of the output characteristic are separated in frequency by an amount determined by the interelectrode capacity of a tube associated with the circuit, thus permitting a maximum sensitivity in response to frequency variation.

It is also a further object of this invention to provide an improved frequency discriminator network which permits eifectively controlling the rise of the low frequency and high frequency peaks of the output response characteristic and the spacing of said peaks, the sharpness of said peaks being substantially independent of variations in input and output loading of the discriminator network.

In a frequency-modulation signal-receiving system, automatic volume control (AFC) or limiter means may be provided preceding the discriminator networks and detector. Hence, it is often desirable to provide AVC potential for the control of signal reception. Accordingly, it is a further object of this invention to provide an improved frequency discriminator network involving a single circuit, seriesand parallel-resonant, which may also provide effective and symmetrical AVC potentials with respect to the mean or center frequency without impairing its operation as a frequency-modulation detector.

The invention will further be understood from the following description when considered in connection with the accompanying drawings, and its scope is pointed out in the appended claims.

In the drawings,

Figure 1 is a schematic circuit diagram of a frequency discriminator network embodying the invention and providing selective tuning means for a radio signal receiving system,

Figure 2 is a similar schematic circuit diagram of a frequency discriminator network embodying the invention, as a modification of the circuit of Fig. 1, including elements for adjusting the tuning and other frequency-response characteristics of the network,

Figure 3 is a further schematic circuit diagram showing a portion of the circuit of Fig. 2 modified to provide symmetrical AVC potentials,

Figure 4 is a schematic circuit diagram of a frequency discriminator network embodying the invention in connection with an intermediate frequency amplifier of a superheterodyne receiver, and modified to include a minimum of circuit elements,

Figures 5, 6 and 7 are graphs showing curves representing certain operating or response char: acteristics of a frequency discriminator network of the type shown in Figs. 1, 2 and 4.

Referring to Fig. 1, the output circuit 8 of a bandpass amplifier 9, connected with an antenna circuit l as a suitable signal source, is terminated by a shunt coupling resistor or impedance ll connected from the circuit 8 to ground l2 through a bypass capacitor IS. The circuit 8 and ground or chassis l2 represent the high and low potential sides respectively of the signal-conveying channel of the system.

A frequency discriminator network I is coupled to the signal-conveying channel for receiving and converting frequency-modulated or variable frequency signals into amplitude-modulated signals, and to derive from the latter audio frequency or control potentials for further utilization in the signaling system.

In the present example, the frequency discriminator network I5 is coupled at its high potential terminal IE to the circuit 8 through a suitable coupling capacitor ll of relatively low capacity, for preventing the transmission of audio frequency hum potentials from the amplifier 9 to the network I5, and is coupled at its high potential output terminal I8 through a suitable audio frequency coupling capacitor ill to an audio frequency system comprising a volume control device 20, an audio frequency amplifi r 2| and a sound-producing output device 22. The audio frequency system and the low potential terminal 24 of the frequency discriminator network l5, as indicated at 23 and 25, respectively, are provided with a common circuit return path which preferably is the chassis or ground. With this arrangement, the low potential side I2 of the signal-conveying channel is also common to the low potential side of the discriminator network and of the audio frequency system.

The discriminator network I5 includes a tuned circuit or section 26 comprising a tuning inductance 21 and shunt capacity means therefor provided by two series-connected capacitors 28. A midtap 29 between the capacitors is connected to ground at 30 through a resistor element 3|, as means for resistance cancellation of losses in the tuned circuit, according to well-known filter design.

One side of the tuned circuit 26 is connected to the high potential terminal I6 of the discriminator network, while the other side is connected to an intermediate terminal 35. A series resonating capacitance 31 is provided by suitable means between the intermediate terminal 35 and the low potential terminal 24 of the discriminator network. This may be the interelectrode capacity of an electric discharge device, such as a rectifier 36, connected between the terminals 35 and 24. A similar electric discharge rectifier device 38 is connected between the high potential terminal iii of the network and the output terminal 18. The network is essentially of the two-terminal type, having the high potential terminal at I6 and the low potential terminal at 24.

An output resistor or impedance element 35 for the rectifier 38, and an output resistance or impedance element 40 for the rectifier 36, being substantially equal in impedance or resistance value, are connected serially between the terminals l8 and 24. A tap connection 4| between the resistors is provided through a circuit lead 42 and a choke coil 43 to the terminal 35.

As will hereinafter appear, this provides a balanced output circuit for the discriminator network and substantially zero D. C. output voltage between the terminals l8 and 24 in response to a fixed carrier wave or signal at the center frequency F0, or substantially zero a-f (audio frequency) voltage for an amplitude-modulated wave at the frequency F0.

For a variation of frequency higher or lower than F0, D. C. potentials of opposite sense are obtained between terminals 18 and 24, providing a resultant output voltage of variable amplitude and polarity between the output terminals I 8 and 24. If the frequency F0 is varied at an audio frequency rate, then at a-f, voltage may be obtained between terminals l8 and 24, of a frequency equal to the rate of variation of Fe, and of an amplitude proportional to the deviation from the mean frequency Fe.

It will -be noted that a single shunt path is provided by the frequency discriminator network between the terminals 16 and 24 across the signal-conveying channel of the system, the said path including the tuned circuit or section 26 and the series capacitance at 31. The circuit 23 provides a parallel-resonant element for the discriminator network and resonates at a frequency above the mean frequency of the discriminator network and of the si nal channel of the system. The tuned circuit 26 is inductively reactive below the mean frequency and is tuned to series resonance below said mean frequency by the series capacity at 31. These frequencies are above and below 'the mean frequency by equal amounts.

The frequency diflerence between the upper and lower resonances is determined by the capacitive reactance at 31 and the inductive reactance of the circuit 26. It may be pointed out that the eflect of a series capacitance, as at 31, for a shunt-tuned circuit, as at 26, is to place the series resonance of 26-31 at a lower frequency than the parallel resonance of 26, and

the frequency difference of the resonances "is substantially the frequency diii'erence in resonanceof 26 which would be obtained by adding 31 in shunt thereto. Therefore, this difference is a minimum when the tube capacity alone at 31 is provided as the series resonating capacity for any given value of inductance at 21.

The rectifiers 36 and 38 are preferably of the diode type, as indicated, having cathodes 45 connected with the terminals l8 and 24, and anodes W connected with the terminals 86 and 35. The cathodes are maintained at substantially the same r-f (radio frequency) or i-! (intermediate frequency) potential by suitable bypass capacitors connected across the resistor or output impedance elements 39 and 40, as indicated at 41. Generally, the output terminal is is maintained at substantially the same r-f potential as the output terminal 24 by means of the r-] bypass capacitors 41. In the present example, this is also at ground potential, since the terminal 24 is connected to ground 25, and to the low side of the signal input channel through ground l2. The choke coil 43 may be connected between the terminal 4| and any point between the terminals l3 and 35, thus providing a D. C. path to the diode plates.

The D. C. circuit through the rectifier device 38 may be traced from the terminal l6 through the device 38 to the terminal 18, thence through the output resistor section 39 to the terminal 4! and returning through the lead 42 and choke coil 43 to the terminal 35 and through the coil or inductance 21 back to terminal I6.

The D. C. path through therectifier 36 may be traced from the terminal 35 through the recuser as to the terminal :4, theme through the resistor section 40 to the terminal 4! and returning through the lead 42 and choke coil 43 to the terminal 35, thus making the rectifier 36 shunt -connected across the output resistor 46 and the voltage source existing between the terminals 35 and 24, while the rectifier 33 is shuntconnected across the output resistor 39 for D. C. potentials and across the voltage source existing between the terminals 96 and 2t for'r-f and i-f potentials.

The choke coil 33 is an 1-1 choke coil provid- The signal voltage existing at the terminal l6 or the plate of diode 38 with respect to the low potential terminal 24 is a minimum at series resonance of the circuit with the capacity 31, as indicated at 50 on the voltage response or frequency discriminator S-shaped curve 5! of Fig. 5, which is drawn with respect to the usual impedance-coupling response characteristic 52 of voltage output with respect to frequency, and

is a maximum at parallel resonance of the circuit 26, as indicated by the peak 53 of curve 5| in Fig. 5.

The signal voltage existing between the terminal or the plate of diode 38 and the terminal 24 is a minimum at parallel resonance of the circuit 26, as indicated at 54 on the frequency discriminator or S-shaped curve 55 of Fig. 6, and is a maximum at series resonance of the circuit 26 with the capacity 31, as indicated by the peak 56 on the curve 55.

Referring further to Figs. 5 and 6, it will be noted that the curve 5| has been applied to the curve 55 in Fig. 6, and that a crossover point 51 is indicated for the frequency F0. The curves of Fig; 6 also indicate the D. C. potential appearing across resistors 39 and 40, as well as the rinput potential at points l6 and 35 with respect to ground. The r-j input across the diode 38 causes D. C. current to flow in the resistor 39. The 1-) input-across the diode 36 causes D. C. current to flow in the resistor 4|? The cathodes ar positive and the center tap 4| is negative. Therefore, the D. C. voltage from terminal [8 to the terminal 24 is the sum of the two voltages, and when the voltage across the resistor 39 equals the voltage across the resistor 40, the voltage from the terminals 18 to 24 will substantially be zero. 7

The two S-shaped voltage-frequency characteristic curves 5| and 55 each include center portions 50--53 and 5456 which are ubstantially linear and provide positive and negative slopes intersecting at 51 at a frequency of the order of the mean frequency, substantially midway of said center portions as indicated in Fig. 6.

Fig. 7 shows the voltage between output terminals l8 and 24 with respect to frequency. Thus, with respect to the curve 58-5960, if the frequency of an applied signal is started at a low frequency and increased, the D. C. voltage I8 to 24 will increase until the frequency F1 is reached and terminal I8 becomes negative with respect to the terminal 24. From F1 to F0 it will decrease and will be zero at F0, and from F0 to F2 it will increase, and terminal 18 will be positive with respect to ground. -From frequency F2, going higher in frequency, the output voltage decreases.

It is the linear region between the peak and the peak 81 which is utilized for such control devices as AFC or frequency indicating devices, and it is readily apparent that the resulting reversal and increase of voltage with any shift from the mean frequency F0 may be used for a control or indicating means. Likewise, any variation of frequency from th mean value of F at an (1-! rate results in an a-j voltage appearing between points I8 and 24, and this a-f voltage 18 hnear with respect to frequency deviation, if the curve 59 is linear and the peaks 85-81 are not exceeded. This function is used as a detector of frequency-modulated signals, as indicated in the circuit diagram.

The band width of the discriminator or frequency variation detector must be equal to the signal band width, such as F1 to F2 or Fm to F2 in Fig. 7. v

For any given value of inductance 1n the circuit 26, the ratio of the capacities at 31 and in shunt with inductance 21 determine the distance between the peaks F1 and F2 of the voltage response curves shown in Figs. 5, 6 and 7. Since the capacity at 31 is that which exists across the electric discharge device or rectifier 36, it is substantially a minimum and, therefore, it will be seen that, with this circuit, the distance between the peaks at F1 and F2 and the mean frequency F0 may be equalized by adjusting the tuning of the circuit 26 as by varying the capacity in shunt with the inductance 21. The band width of the discriminator or frequency variation detector must be equal to that of the incoming signals. Hence, a shunt capacity as at 8| (see Fig. 2) is often provided for setting the band width of the network.

Referring more particularly to Fig. 7 along with Fig. 1, it has been found that the series-parallelresonant circuit 26-3'| may be tuned over a relatively wide frequency range, fo example, from a center or mean frequency F0 to a center or mean frequency Fa, substantially without varying the amplitude of the peaks indicated at BI and 62 for the new low and high frequency limits F4 and F5. Thus, the inductance 21 may be provided with a movable tuning core 63, preferably of ferromagnetic material and controlled by suitable means, such as a tuning dial 64, for varying the tuning of the circuit 26-31 and of the system, through a. predetermined tuning range as established by the band pass amplifier 9. Thus, in an r-f system, the circuit 26-31 may provide the sole tuning means for the receiving channel. It is obvious, however, that the ampliher 9 or other input system may be variably tunable, if desired.

With regard to the circuit elements employed for tuning in the range of 6.5 mc. (megacycles) the variable inductance 2] provided with a movable ferromagnetic core may, for example, have 16 turns of No. 30 wire on a one-half inch form and the capacitors 28 may have a value of 200 mi. (microfarads). The capacity at IT may be of the order of 200 mmf. (micromicrofarads) with a coupling resistor of 2200 ohms at I I, while the output resistor sections 39-40 ma have a value each of 100,000 ohms, shunted by capacitors 41 of 250 mmf. The output impedance at 20 is preferably relatively high and of the order of 1 to 2 megohms. The tubes 36 and 38 may be of the type commerciall known as the RCA 6H6 double diode.

Referring now to Fig. 2, the circuit arrangement is similar to that of Fig. 1, and like circuit elements are designated by the same reference numerals.

The discriminator network includes the rectifier devices 36 and 38, connected to high and low potential terminals l6 and 35 and to output terminals 24 and I8, across resistor elements 39 and 40 shunted by capacitors 41, as in the cirlt of Fig. 1, and the center tap 4| is connected through the circuit 42 to the terminal 35 as a D. C. return path for the two rectiflers.

However, in the present example, a, variable resistor element 10 is provided in the lead 42 and the parallel-resonant circuit 26 comprises a high frequency inductance II and variable shunt capacitor 12. Signals are applied to the discriminator network through the coupling capacitor I! from a variable resistor I3 connected with the output circuit 14 of an amplifier or limiter stage 15 having signal input terminals 16 and 11.

It will be noted that the ground connection for the discriminator circuit is provided at the terminal 4| as indicated at 18, thereby providing a balanced output circuit connection for output terminals 19 and 80 with the terminals II and 24, respectively, for push-pull operation of apparatus (not shown) which may be connected with the terminals I9 and 80 as utilization means for the converted signal.

It will be noted also that a variable capacitor 9| is provided in shunt with the rectifier 36, thus being an addition to the internal electrodal capacity of the tube 36. Referring to Fig. '1, it has been found that variation of this capacity may serve to shift the low frequency peak 85 of the curve 58-59-40 to a lower value as indicated at 86, an increase in capacity serving to shift the peak in the direction of the lower frequency. Thus, variation of the band width of the response of the network may be provided by the capacitor 8|, as a single element. The change in the band width may be made without affecting the desired symmetry of the response curve and wtihout materially shifting the high frequency peak F2, as will be seen from inspection of Fig. 7.

It has been found that the larger the shunt capacity 'IZ-in the parallel-resonant circuit with respect to the capacity 8| providing the seriesresonant controlling element, the closer together are the peaks and 81 of Fig. '7. By maintaining the ratio of the capacity 12 to the capacity 8|, including stray and diode capacity, and by maintaining the Q of the coil, the sensitivity may be increased by reducing both capacities l2 and 8| and increasing the inductance of the winding 1| for the same band width.

It has further been found that the input and output loading, represented by the variable resistors l3 and 10, respectively, does not materially alter the sharpness of the peaks 50 and 54 of Fig. 6, and therefore the return traces 56 and 60 of the curve 58596|l of Fig. 7 are relatively sharp as indicated, being sharper than known discriminator frequency networks in this respect.

Variation of the resistor 13 causes a variation in the input impedance or loading of the discriminator network and controls the rise of the high frequency peaks 53 and 81 of Figs. 5, 6 and 7, while the variation of the output load by variation of the resistance 10 controls the rise of the low frequency peaks 56 and 85 of Figs. 6 and 7. In any'case, however, the impedance or resistance value of the D. C. return path through circuit 42 and the resistor 10 must be relatively low, or the audio frequency output voltage will be reduced, for the reason that resistor 10 is effectively in series with the resistors 39 and 49 in the output circuit. Therefore, the resistor 14 should be a. small percentage of the resistance of the elements 39 and 40.

While two rectifiers provide higher output potentials and a higher degree of sensitivity together with a balanced audio frequency output, one rectifier may be used; that is, either rectifier may be omitted, if the normal D. C. outputvoltage in its output resistor section is maintained and the capacitor 8| is adjusted to compensate for the capacitance of the tube 36. This is for the reason that each rectifier response is an s-curve rather than a resonance curve, as hereinbefore pointed out.

Referring to Fig. 3, in which the same reference numerals are provided for the same circuit elements as in Fig. 2, it will be noted that AVC potentials may be derived from the circuit by utilizing the resistor of Fig. 2 and the choke coil of} Fig. 1 in series in the return circuit lead 42' from the center tap M to the intermediate terminal 35, the resistor being indicated at 82 and the choke coil at 83, with the AVCconnection taken at a point between the two elements through a filter resistor 88 for the AVG lead 86.

To increase the audio frequency output, the resistor 62 is provided with an audio frequency bypass capacitor 84, thus reducing the impedance to a-f signals. Since the bypass 64 also reduces the impedance at the signal frequency, the choke coil 83 is provided to prevent shunting of the r-f or 1-! voltage appearing across the diode rectifier 36. The AVC voltage thus obtained is symmetrical with respect to F and may be substantially constant over the frequency range l b-Fa.

Referring to Fig. 4, in which the same reference numerals are applied to the discriminator circuit as appear in common with the preceding figures, the discriminator network l5 comprises the two rectifier devices 36 and 36 connected with the terminals I 6 and 35 and with the terminals 08 and 24. Between the terminals I 8 and 24, the output resistor sections 39 and 46 are serially connected with a single bypass capacitor 65 in resonates at the higher frequency. In practice. the tuning ofgthe circuit 26 is adjusted to provide the resonant peaks on opposite sides of the mean frequency of the network, the spacing of the peaks being determined by the ratio of the capacity 9'! to the capacity 31.

The signal supply for the discriminator is taken from an 1-! amplifier stage Hill which may also be the limiter stage of the H amplifier, having input terminals I M and having a tuned output circuit I62 coupled to the discriminator circuit i5 through a coupling capacitor I63. A shield elemer t; i 04 may be provided to prevent stray capacity o shunt therewith, thus eliminating the choke coil I and one bypass capacitor of the preceding circuits, and the center tap H is connected through a direct lead connection 90 with the terminal 35. However, this connection may be returned to any portion of the circuit between the terminal 35 and the terminal i6 and is shown connected to the terminal 35 only by way of example as being a preferred arrangement of the return' connection for the D. C. path through each rectifier. The output terminal I8 is connected through an output filter 9| to an output terminal 62, and the return circuit is provided from an output terminal 93 through ground 94 to the terminal 24, thus applying to the terminals 32 and 93 the resultant output potentials appearing across the output terminals l8 and 24, as described in connection with the preceding embodiments. The filter 9| is provided to attenuate or de-emphasize a-f signals in the upper end of the audio frequency range to compensate for the higher audio frequency boost or increase provided in the usual signal at the transmitter. Between terminals i6 and 35 there is provided the tuned parallel-resonant circuit 26 comprising an inductance winding 96 and shunt capacitor 9'8, the inductance being suitably adjustable as by means of a movable tuning core 68. The capacity existing across the electrodes of' the tube 36 is indicated at 31 and may be utilized as the capacity element for tuning the shunt path through the circuit 26 from the terminal it to the terminal 24, to resonance at the lower frequency, while the shunt-tuned circuit 26 magnetic coupling between the circuits 26 and I02. The effect of the tuned circuit I02 across the signal channel, indicated at I05 and H16 as the low and high potential sides thereof respectively, is to raise the amplification or to increase the sensitivity of the amplifier stage preceding the discriminator network and to further increase the selectivity or sharpness of the return lines 56-60 of the response characteristic of the network shown in Fig. 7.

The operation of the circuit of Fig. 4 is other wise the same as that of the preceding figures and has the advantage of not onlyhigher gain or sensitivity but also improved selectivity with fewer circuit elements. As shown, the circuit is particularly adapted for use as the second detector of a. superheterodyne receiver for'frequency modulation signal reception. The voltage between the terminals I 8 and 24 is made substantially the same by connecting between said terminals the bypass capacitor 95 which also forms part of the filter iii. The voltage at the terminal I 6 with respect to ground is minimum at the resonant frequency provided by the series resonance of the circuit 26 and the capacity 31. It is a. maximum at that terminal with respect to ground, at parallelresonance of the portion 26 of the circuit 26-31.

The voltage at the terminal 35 with respect to ground is at a minimum at parallel resonance of vention may provide a single shunt path across a signal circuit or transmission channel of a receiving system including therein inductance and shunt capacity means providing parallel reso-' nance above a mean frequency, and means, preferably including the electrode capacity of an electric discharge device, forming a series capacity in the circuit resonating with the inductive reactance of the parallel-resonant portion to tune the network below said mean frequency, thereby providing a frequency discriminator network While the invention has been described in its several preferred embodiments as particularly adapted for the detection of frequency-modulated signals, it is obvious that it may be. utilized in connection with any signal transmission channel subject to the frequency variation of a received signal and that an improvement in sensitivity, selectivity and stability over that of known frequency discriminator circuits may be obtained, particularly in connection with high and ultra high frequency signals. In addition, the network provided is essentially of the two-terminal type, including but one tuned circuit and a minimum of circuit elements,

I claim as my invention:

1. The combination of a circuit responsive to signals in a frequency band substantially centered about a predetermined frequency, a frequency variation detector comprising a two-terminal network connected across said circuit, and means in said network providing parallel resonance at a higher frequency and series resonance at a lower frequency by an equal difference with respect to said predetermined frequency.

2. The combination of a circuit responsive to variable frequency signals in a frequency band substantially centered about a predetermined frequency, a frequency discriminator network providing a single tuned shunt signal path across said circuit, and tuning means connected serially in said path providing parallel resonance at a higher frequency and series resonance at a lower frequency by an equal difference with respect to said predetermined frequency.

3. A circuit responsive to a band of frequencies centered about a carrier wave the frequency of which is modulated in accordance with a signal, a network connected across said circuit providing a single signal path tuned for parallel resonance above and series resonance below the carrier frequency by an equal frequency difference, and rectifying means connected with said network for deriving therefrom signal and control potentials proportional to the frequency variation of said wave.

4. In a signal transmission system, a frequency discriminator network including series and parallel-resonant elements in a single shunt signal path providing a voltage-frequency output characteristic having positive and negative peaks, and means in said network for causing said peaks to be separated in frequency above and below a mean frequency by predetermined equal amounts and for establishing a predetermined band width and sensitivity in said network.

5. In a signal transmission system, a frequency discriminator network including a parallelresonant circuit in a single shunt signal path providing a voltage-frequency output characteristic having positive and negative peaks, a rectifier in said path having inherent capacity providing series resonance with said circuit below a predetermined frequency, means in said network for causing said peaks to be separated in frequency by a predetermined amount above and below said frequency, means for varying one of said parallel-resonant elements to vary the tuning of said network over a predetermined frequency range, and means for maintaining the amplitude of said peaks and the band width of said network substantially constant.

6. In a signal transmission system, a two-terminal frequency discriminator network having a high potential terminal and a low potential terminal and comprising a parallel-resonant circuit including inductance and shunt capacity means connected to the high potential terminal,

and rectifier means having inherent capacity providing series resonating capacitance for said network in series with said parallel resonant circuit between said circuit and the low potential terminal, said parallel-resonant circuit being tuned to a frequency higher than a predetermined reference frequency by a desired frequency value, and said capacitance tuning said parallelresonant circuit to a frequency lower than said reference frequency by said desired frequency value.

'7. The combination with a circuit responsive to variable frequency signals in a frequency band substantially centered about a predetermined frequency, of frequency discriminator network providing a single tuned shunt signal path across said circuit, means in said network including a variably tunable circuit providing parallel resonance at a higher frequency and series resonance at a lower frequency with respect to said predetermined signal frequency, and means for varying the tuning of said circuit to shift the discriminator frequency response over a relatively wide frequency range, thereby to provide continuously variable tuning of said system through a relatively wide frequency range for the reception of frequency-modulated signals and the like.

8. In a signal-conveying channel, the combination of a band pass amplifier having an output circuit, a frequency discriminator network providing a single shunt signal path across said output circuit, a parallel-resonant circuit in said shunt path, capacitance means in series with said parallel-resonant circuit effective to render the shunt path series resonant, and means for variably tuning said parallel-resonant circuit through a predetermined frequency range within the pass band of said amplifier.

9. The combination with a signal amplifier having an output impedance element, of a frequency discriminator network connected substantially in shunt therewith and including a parallel-resonant circuit having inductance and shunt capacity elements, and a rectifier device connected with one side of said circuit providing a series resonating capacity element for said circuit, a second rectifier device and said output impedance element connected with the opposite side of said resonant circuit, an output impedance element for each of said rectifier devices connected serially between said devices, means providing a common return circuit connection between said impedance elements and said rectifier devices, the impedance of said last-named connection being relatively low with respect to each of said impedance elements, and a balanced out put circuit connected across said impedance elements.

10. The combination with a signal amplifier having an output impedance element, of a frequency discriminator network connected substantially in shunt therewith and including a parallelresonant circuit having inductance and shunt capacity elements, and a rectifier device having a cathode and having an anode connected with one side of said circuit providing a series resonating capacity element for said circuit, a second rectifier device having a cathode and having an anode connected with the opposite side of said resonant circuit, means providing a coupling connection between said output impedance and said opposite side of the last named tuned circuit, an output impedance element for each of said rectifier devices connected serially between the cathodes of said devices, means providing a common return circuit connection between said impedance elements and the anode side of said rectifier devices, the impedance of said lastnamed connection being relatively low with respect to each of said impedance elements, and means for maintaining the cathodes of said rectifier devices at substantially the same potential.

11. The combination with a signal amplifier having an output impedance element, of a frequency discriminator network connected substantially in shunt therewith and including a parallel-resonant circuit having inductance and shunt capacity elements, and a rectifier device connected with one side of said circuit providing a. series-resonating capacity for said circuit, a second rectifier device and said output impedance connected with the opposite side of said resonant circuit, an output impedance element for each of said rectifier devices connected serially between said devices, means providing a common return circuit connection between said impedance elements and said rectifier devices, means for varying the impedance of said last-named connection and the output impedance of said amplifier thereby to vary the output and input loading of said network and the amplifier of the high 12. A frequency variation detector comprising a pair of input terminals upon which are impressed frequency modulated carrier energy having a predetermined mean frequency, a twoterminal network connected between said input terminals, said network consisting of a tuned circuit parallel resonant to a frequency above said means frequency, and a capacitative element series resonating the inductive reactance of said tuned circuit to a frequency below said mean frequency by a substantially equal frequency value, and utilizing means coupled to opposite terminals of said two-terminal network.

13. In a detector of frequency modulated carrier wave energy, a parallel resonant input circuit tuned above the mean frequency of said energy by a predetermined value, a pair of opposed diodes, like electrodes of the diodes being connected to respective opposite sides of said parallel resonant circuit, a resistive load impedance connecting the remaining like electrodes of the diodes, a conductive connection from an intermediate point on the load impedance to one side of the input circuit, means establishing a point of the load impedance at an invariable alternating potential, and a capacitative element, in series between said input circuit and said point, series tuning said input circuit to a frequency less than said mean frequency by substantially said predetermined value.

JOHN D. REID. 

